After Fred Steese spent two decades in a Nevada prison for murder, evidence indicating that he was innocent was found buried in the prosecution’s files. It was proof that Mr. Steese, as he’d always claimed, had been hundreds of miles away on the likely day of the murder and couldn’t have been the killer.

In Maryland two years earlier, the conviction of James Thompson, who had also served 20 years for murder and rape and whose case involved police and prosecutorial misconduct, was thrown into overwhelming doubt when his DNA didn’t match the semen found in the victim.

In neither case did prosecutors jump to set the prisoner free. Instead they vowed to retry the men unless they agreed to a plea bargain called an Alford plea, in which the defendant enters a guilty plea while also asserting his innocence for the record. The deal allows the inmate to leave prison right away. But he remains convicted of the crime, forever a felon.

Another strategy prosecutors employ is to offer to convert an inmate’s sentence to time served but with his conviction intact. Freedom in both instances is dangled as a carrot to entice the defendant to give up on exoneration.